Cutting inserts in modern cutting tools, such as in turning bars and different milling cutter bodies, are preferably of the so called indexable cutting insert type, i.e. they are not irreversibly fastened in the holder, but can be detached and turned in order to indexate a new operative cutting edge, when the preceding edge has attained a certain degree of wear, or is replaced when all cutting edges have been worn out. In order to enable this indexating or this replacement, the indexable cutting insert is clamped in the holder by a screw through a hole in the very cutting insert, a detachable clamp presses upon the upper side of the cutting insert, and a locking pin presses against the hole wall of the cutting insert, etc. The most common clamping arrangement would however be a locking screw, which is threaded through a center hole in the indexable cutting insert and is screwed firmly into a threaded hole of the cutting insert seat of the holder. A drawback with this device is however that the screw must be wholly unscrewed and be detached from the holder, when it is time for a replacement of the cutting insert. This may be very inconvenient and time consuming, partly because it can comprise a great number of replacements of cutting inserts, partly because the screws are small and unmanageable, which entails that one easily drops them and that they easily get lost.
A number of solutions have been suggested to overcome the drawbacks with loose screws when replacing cutting inserts. Hence, there are locking screws disclosed in U.S. Pat. No. 3,341,921, SE-C-447 879, EP-A-51 775 and DE-A-3 118 467, all at which are based on the principle, that the center hole of the cutting insert is as large as to permit the wholly round screw head to pass through it. When the cutting insert is clamped, only a part of of the lower side of the head presses against the corresponding circumference section of the hole surface. Although these designs usually function satisfactorily, they are impaired with certain drawbacks. Thus, as mentioned, the clamping force is concentrated to a short, theoretically seen point or line-shaped abutment area between the screw and the cutting insert, which entails large bending strains upon the screw, which in turn results in that the latter is deformed at too strong a tightening. Furthermore, this construction is not wholly reliable at large cutting forces (i.e. at high feeds or laarge cutting depths), particularly not at large relief angles.
In U.S. Pat. No. 3,913,197, there is shown a locking screw, which is relatively severely angled in relation to the normal of the cutting insert, the screw head being brought to press against two diametrically opposed surfaces or points in the cutting insert hole. If the screw is somewhat untightened, the cutting insert can be removed. However, this construction is directly inappropriate for positive cutting inserts and is only adapted for cutting inserts whose side surfaces form straight angles to the cutting insert base surface (so called neutral or negative cutting inserts).
Further, in U.S. Pat. No. 4,430,031 a locking screw for cutting inserts is disclosed, the portion of the locking screw in the center hole of the cutting insert being eccentrically bent outwards and of a smaller cross-section than the hole's cross-section, whereby an untightening of the screw by about half a revolution enables a detaching of the cutting insert from the cutting insert seat. A drawback of this construction is that the operator may easily tighten the screw too strongly, so that the maximal radial bending-out of the screw head is exceeded and the head returns radially inwards, whereby the clamping becomes insufficient.